A membrane bioreactor is known, for example, for purifying waste water, and comprises a basin which is partially filled with active sludge. During operation, the waste water is fed to the basin where it mixes with the sludge. The active ingredients in the sludge take care of purifying the waste water. This process is accelerated even more by supplying a gas, usually air, from below to (part of) the basin. The mixture of sludge and waste water is then fed to the membrane filtration module where purified water is discharged as permeate, while the retained liquid, polluted particles and sludge particles as retentate are fed back to the basin. The membrane bioreactor is able to work with a high concentration of sludge particles, especially compared to a conventional system in which the bioreactor is combined with a settling tank. As a result, the discharged purified water can be of high quality, and it is even readily possible to use the membrane bioreactor for treating heavily polluted sewage water and/or streams of industrial waste water.
The known membrane bioreactors can be divided into two groups, i.e.: a dry-pit system or a submerged system. With the dry-pit system, a membrane filtration module is placed outside the basin of the bioreactor. With the submerged system, membranes are suspended inside the basin of the bioreactor. In recent years, both systems have developed in such a way that they show an increasing number of similarities. Thus, for example, the membranes of the submerged system are more and more often accommodated in a housing provided with inlet and outlet apertures, which housing is then suspended in the basin like a box. In addition, there is a development taking place where more and more facilities are being placed around these boxes which are intended to supervise the flow past these membranes in order to optimize the performance of these membranes. This has resulted in a membrane filtration module for the purpose of the submerged system which is or will be increasingly similar to a membrane filtration module in the dry-pit system.
An example of a submerged system with flat membrane panels in a treatment tank is disclosed in EP 0 510 328. An example of a dry-pit system with tubular membranes which are accommodated in a membrane filtration module is disclosed in U.S. Pat. No. 5,494,577.
For both systems, it is disadvantageous that the membrane surfaces can become soiled quickly and that the flow passages inside the membranes and/or between and/or around the membranes often become blocked with foreign particles in the liquid stream during operation. This soiling and/or these blockages are caused by all kinds of particles which are entrained with the waste water, such as hairs, threads, etc. The soiling and/or the blockages may also be caused by biologically, physically or otherwise deformed particles which result from the reactions between the sludge and the waste water. Another possibility is that soiling may precipitate or blow or otherwise end up in the basins which are usually open to the elements. The direct consequence of the soiling and/or the blockage of the flow passages is the loss of effective membrane surface. In addition, it results in the distribution of the liquid stream across the flow passages no longer being homogeneous. This non-homogeneous distribution leads to large variations in the liquid velocity and the turbulence thereof along the flow passages, as a result of which a crust of particles may form along the membrane surfaces. This in turn leads to a greater risk of blockage of (a part of) the flow passages, as a result of which the liquid distribution may become disturbed even further. As a result, an increasing amount of effective membrane surface is lost and an increasing amount of energy has to be supplied in order to maintain the through-flow through the flow passages which are increasingly difficult to flow through.
In order to prevent the flow passages from becoming soiled and/or blocked, it is known to use a filter upstream of the membrane filtration module in order thereby to catch particles. However, it has been found that thread-like particles are still able to slip through the filter, and then still cause the abovementioned problems. Furthermore, it has been found that the encrusted particles in the membrane filtration module are very difficult to remove, and that they can damage the membrane material. If soiling is observed with the known systems, then the soiled membrane filtration module is disconnected, connected to a cleaning unit and cleaned manually or semi-automatically. In this case, it is common practice for the blocked flow passages to be flushed back regularly with a cleaning liquid. This procedure usually takes up 10 to 20% of the operating time of a membrane filtration module. The possibility and the frequency thereof is dependent on the type of membrane filtration module and is usually in the order of magnitude of one to 60 minutes. In addition, the membrane filtration module can also be cleaned using a chemical cleaning liquid. This entire process takes as much as half an hour to a few hours per membrane filtration module and is again carried out at a frequency of once a day to once a week, or once a month to once a year, depending on the membrane filtration module and operation. If desired, the membrane filtration module can be opened, the blocked membranes can be removed therefrom and the soiling can then be removed using brushes, jets of water or any other mechanical ancillary means. This cleaning method takes even more process time and is generally very labour-intensive and is only carried out in cases of extreme soiling and/or blockage. If, in addition, a filter having openings smaller than 5 mm, or more commonly smaller than 3 mm and preferably smaller than 1 mm is used upstream of the membrane filtration module, then this filter has to be cleaned very regularly. An apparatus for treatment of a fluid is known from DE 2196 20 246.